1. Field of the Invention
This invention relates to a method of and a system for reading out image signal, and more particularly to a method of and a system for reading out image signal in which the density of picture elements of the read-out image signal can be changed.
2. Description of the Related Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, xcex1-rays, xcex2-rays, xcex3-rays, cathode rays or ultraviolet rays, they store a part of the radiation. Then when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is generally referred to as xe2x80x9ca stimulable phosphorxe2x80x9d. In this specification, the light emitted from the stimulable phosphor upon stimulation thereof will be referred to as xe2x80x9cstimulated emissionxe2x80x9d. There has been known a radiation image recording and reproducing system in which a sheet provided with a layer of the stimulable phosphor (will be referred to as xe2x80x9ca stimulable phosphor sheetxe2x80x9d, hereinbelow) is first exposed to a radiation passing through an object such as the human body to have a radiation image of the object stored on the stimulable phosphor sheet, a stimulating light beam such as a laser beam is caused to scan the stimulable phosphor sheet so that the stimulable phosphor sheet emits stimulated emission as signal light bearing thereon information on the radiation image, the stimulated emission is photoelectrically detected, thereby obtaining an analog image signal, the analog image signal is sampled at predetermined intervals and quantized, thereby obtaining a digital image signal at a predetermined picture element density, and the radiation image of the object is reproduced as a visible image on the basis of the digital image signal on a recording medium such as a photographic film or a display such as a CRT. See, for instance, Japanese Unexamined Patent Publication Nos. 55(1980)-12429, 56(1981)-11395 and 56(1981)-11397.
This system is advantageous over a conventional radiography system using silver salt film in that an image can be recorded over a very wide radiation exposure range.
As a system for reading out the stimulated emission, there has been proposed a radiation image read-out system in which a photoelectric read-out means is disposed on each side of the stimulable phosphor sheet, stimulating light is projected onto one side or both sides of the stimulable phosphor sheet and the stimulated emission from both sides of the stimulable phosphor sheet is detected by each of the photoelectric read-out means. See, for instance, Japanese Unexamined Patent Publication No. 55(1980)-87970. In such a radiation image read-out system, since a single radiation image is stored in a stimulable phosphor sheet and a pair of photoelectric read-out means are disposed to detect the stimulated emission from both sides of the stimulable phosphor sheet, light collecting efficiency is improved, and by adding the obtained two image signals at a predetermined ratio of addition, positions in which noise components are detected differ by the sides of the stimulable phosphor sheet and accordingly, an addition signal which is improved in S/N ratio as compared with an image signal obtained from only one side can be obtained.
Further there has been proposed a method of superposing images in which an addition image signal is obtained after carrying out filtering processing, using a filter having frequency response properties such as will increase the S/N ratio of an image signal (including an addition signal), on a single image signal obtained from one side of the stimulable phosphor sheet or two image signals obtained from opposite sides of the same. (Japanese Unexamined Patent Publication No. 7(1995)-287330) In accordance with this method, since the amount of exposure to the radiation to which the object is exposed upon taking the radiation image is obtained and the parameter (the coefficient of filter) which is used for carrying out filtering processing is obtained on the basis of the amount of exposure to the radiation, an image signal representing an image of optimal quality or an addition image signal representing a superposed image of optimal quality can be obtained according to the amount of exposure to the radiation. Further, since processing to change the frequency characteristics is carried out on the overall image signal, it becomes unnecessary to effect frequency transformation such as Fourier transformation and the amount of computation can be reduced.
There has been a demand toward changing the density of picture elements of a single image signal or an addition signal in the image read-out section of the aforesaid radiation image recording and reproducing system or in the radiation image read-out system.
For example, when a large number of radiation images are taken as in a group examination, there is a demand toward increasing the radiation image read-out speed while the quality of the images to be reproduced need not be so high provided that whether reexamination is necessary can be judged. In such a case, the images need not be read out at a high picture element density. Conversely, there is a case where the image should be read out at a high picture element density so that the image can be reproduced at a high quality even if the read-out speed is lowered.
To read out the image at a picture element density other than the preset picture element density may be realized by simply changing the main scanning speed and the sub-scanning speed of the stimulating light beam. The main scanning speed of the stimulating light beam can be changed by changing the driving speed of the scanning optical system for causing the stimulating light beam to scan the stimulable phosphor sheet (e.g., a polygonal mirror or a galvanometer mirror).
However, when the driving speed of the scanning optical system is changed, it takes a certain time for the driving to be stabilized due to inertia of the optical system, and the optical system cannot be constantly stably driven over the entire driving speed range. Accordingly, it is preferred that the picture element density be changed without changing the main scanning speed of the stimulating light beam. Further when the picture element density is to be changed, it is necessary to change the picture element density in the sub-scanning direction in the same proportion as the picture element density in the main scanning direction.
Further when the picture element density is to be changed, it is preferred that the picture element density changing processing be carried out at a speed as high as possible.
Further, when the picture element density is simply changed, there is a possibility that the following problems arise.
That is,
1) When the picture element density is changed, energy of signal light emitted from the stimulable phosphor sheet per one picture element differs from that for the original picture element density. Accordingly, when the difference between the changed picture element density and the original picture element density is large, the density (or brightness) of the overall image to be reproduced can be changed, which can adversely affect diagnostic performance of the image.
2) When shading correction is to be carried out on the analog image signal, properties of shading to be corrected varies before and after the picture element density change and the shading sometimes cannot be properly corrected.
3) When the analog image signal is to be logarithmically amplified, the frequency transfer properties can vary before and after the picture element density change.
4) When filtering for removing aliasing noise is to be carried out prior to sampling the analog image signal, aliasing noise sometimes cannot be properly cut since Nyquist frequency varies before and after the picture element density change.
These problems arise not only when an image signal representing a radiation image is read out from a stimulable phosphor sheet but also, for instance, when a medium on which an image is printed is scanned by a light beam and light reflected from the medium according to the image printed thereon is read out as signal light.
In view of the foregoing observations and description, the primary object of the present invention is to provide a method of and system for reading out an image signal in which the picture element density can be changed without changing the main scanning speed of the read-out light beam.
Another object of the present invention is to provide a method of and system for reading out an image signal from signal light emitted from both sides of a recording medium in which the picture element density can be changed without changing the main scanning speed of the read-out light beam.
The method of and the system for reading out an image signal in accordance with the present invention are characterized in that a digital image signal having a desired picture element density is obtained without changing the main scanning speed by changing the sub-scanning speed and the sampling intervals.
That is, in accordance with a first aspect of the present invention, there is provided a method of obtaining, in a method of reading out a digital image signal at a predetermined picture element density by causing a light beam to repeatedly scan a recording medium bearing thereon an image in a main scanning direction at a predetermined main scanning speed while moving the recording medium in a sub-scanning direction substantially perpendicular to the main scanning direction at a predetermined sub-scanning speed, thereby two-dimensionally scanning the recording medium with the light beam, photoelectrically detecting signal light emitted from the recording medium upon exposure to the light beam to obtain an analog image signal, sampling the analog image signal at a predetermined intervals, and quantizing the sampled values, a digital image signal at a desired picture element density different from the predetermined picture element density, the method comprising the steps of
changing the sub-scanning speed to m( greater than 0) times said predetermined sub-scanning speed, and changing the intervals at which the analog image signal is sampled to intervals n( greater than 0) times said predetermined intervals.
In a preferred embodiment, the method further comprises a step of carrying out, on the digital image signal, picture element density changing processing for changing the number of the picture elements in the main scanning direction to a/m (a greater than 0) times and the number of the picture elements in the sub-scanning direction to a/n times.
In accordance with a second aspect of the present invention, there is provided a method of obtaining, in a method of obtaining an addition image signal at a predetermined picture element density by causing a light beam to repeatedly scan a recording medium bearing thereon an image in a main scanning direction at a predetermined main scanning speed while moving the recording medium in a sub-scanning direction substantially perpendicular to the main scanning direction at a predetermined sub-scanning speed, thereby two-dimensionally scanning the recording medium with the light beam, photoelectrically detecting signal light emitted from both sides of the recording medium upon exposure to the light beam to obtain two analog image signals, sampling the analog image signals at a predetermined intervals, quantizing the sampled values, and adding two digital image signals thus obtained, an addition image signal at a desired picture element density different from the predetermined picture element density, the method comprising the steps of
changing the sub-scanning speed to m( greater than 0) times said predetermined sub-scanning speed, and changing the intervals at which the analog image signal is sampled to intervals n( greater than 0) times said predetermined intervals, thereby obtaining two intermediate digital image signals, and carrying out, on the intermediate digital image signals, picture element density changing processing for changing the number of the picture elements in the main scanning direction to aim (a greater than 0) times and the number of the picture elements in the sub-scanning direction to a/n times.
In the methods of the first and second aspects of the present invention, the recording medium may be a stimulable phosphor sheet used in the aforesaid radiation image recording and reproducing system as well as a reflection original such as a photographic print and a transparent original such as a photographic film. Accordingly, the signal light emitted from the recording medium includes stimulated emission emitted from a stimulable phosphor sheet upon exposure to the light beam, reflected light reflected by a reflection original, and transmitted light from a transparent original.
As a method of changing the sampling intervals, a method in which a clock frequency-divided from a reference clock for determining said predetermined sampling timings are made and the analog image signal is sampled on the basis of the frequency-divided clock, a method in which a new clock different from the reference clock in cycles are made by use of a PLL and the analog image signal is sampled on the basis of the new clock, or a method in which a plurality of sampling clocks which are different from each other in cycles are prepared and the analog image signal is sampled on the basis of one of the clocks may be employed.
As a method of obtaining the addition image signal, as well as a method in which the image signal components of the two digital image signals for the corresponding picture elements are added together, a method in which the two digital image signals are added together after they are subjected to filtering processing by use of a filter having frequency response properties such as to increase the S/N ratio of the addition image signal as disclosed, for instance, in Japanese Unexamined Patent Publication No. 7(1995)-287330 may be employed.
The picture element density changing processing may be carried out, for instance, by effecting one-dimensional mask operation in the main scanning direction of the image signal, by thinning picture elements according to the desired picture element density, by high-order interpolation such as B-spline interpolation or cubic spline interpolation (disclosed, for instance, in Japanese Unexamined Patent Publication Nos. 8(1996)-16767 and 9(1997)-321981), or by linear interpolation (disclosed, for instance, in Japanese Unexamined Patent Publication No. 9(1997)-50516).
It is preferred that a parameter for the one-dimensional mask operation, a parameter for thinning the picture elements or a parameter of an operation expression for the interpolation be changed according to the values of m and n. The parameter for the one-dimensional mask operation is a coefficient of mask, and the parameter for thinning the picture elements is the intervals at which the picture elements are thinned. The parameter of the operation expression for the interpolation represents the operation expression to be employed in the interpolation (e.g., B-spline interpolation, Cubic spline interpolation or linear interpolation).
In the method of the second aspect of the present invention, it is preferred that the picture element density changing processing be carried out on said two intermediate digital image signals before they are added together.
Further in the methods of the first and second aspects of the present invention, it is preferred that at least one of the following properties be changed according to the values of said m and n.
(1) The beam diameter of the light beam.
(2) The power of the light beam.
(3) The sensitivity of detecting the signal light.
(4) The preset data for shading correction when shading correction is to be carried out on the analog image signal.
(5) The timing at which the data for shading correction is output from a memory.
(6) The frequency transfer properties when the analog image signal is to be logarithmically amplified.
(7) The cut-off frequency when filtering for removing aliasing noise is carried out prior to sampling the analog image signal.
(8) The parameter for filtering processing in the picture element density changing processing (including the parameter for thinning the picture elements when thinning processing is carried out as the filtering processing).
It is not necessary that all of the items (1) to (8) are changed according to the values of said m and n, but at least one of the items (1) to (8) may be changed. For example, when the shading correction need not be carried out, the items (4) and (5) need not be included.
The items (1) to (3) to be changed according to the values of said m and n are for overcoming the aforesaid problem 1), that is, when the difference between the changed picture element density and the original picture element density is large, the density (or brightness) of the overall image to be reproduced can be changed, which can adversely affect diagnostic performance of the image. Specifically, when the picture element density is increased, (1) the beam diameter of the light beam is reduced, (2) the power of the light beam is weakened, and/or (3) the sensitivity of detecting the signal light is increased. To the contrast, when the picture element density is reduced, (1) the beam diameter of the light beam is enlarged, (2) the power of the light beam is increased, and/or (3) the sensitivity of detecting the signal light is lowered.
Similarly when the picture element density is increased, (6) the frequency transfer properties are widened to a high-frequency band, and (7) the cut-off frequency is shifted toward the high-frequency side. When the picture element density is reduced, (6) the frequency transfer properties are narrowed toward the low-frequency side, and (7) the cut-off frequency is shifted toward the low-frequency side.
The item (8) may be changed, for instance, in the following manner. That is, the parameter for filtering processing (the coefficient of the mask operation) in the picture element density changing processing is changed so that the cut-off frequency for mask operation is shifted toward the low-frequency side as the picture element density is reduced. When the parameter for thinning processing is applied, the parameter is changed so that the cut-off frequency is shifted toward the high-frequency side as the picture element density is reduced.
The item (4) is not shifted qualitatively according to the picture element density. Accordingly, by preparing a plurality of sets of data for shading correction for a plurality of representative picture element densities, and data for shading correction corresponding the desired picture element density is selected from the sets of data, or when there is prepared no data for picture element density equal to the desired picture element density, the data for shading correction corresponding the desired picture element density is obtained by interpolation by use of two sets of data in the prepared sets of data.
The item (5) is applied when shading correction is carried out on the analog image signal in real time, and by outputting prepared data for shading correction at a timing according to the desired picture element density, shading correction is carried out on an obtained analog image signal. Specifically, when the picture element density is increased, the sampling speed is increased and accordingly, the data output timing is advanced according to the sampling speed. To the contrast, when the picture element density is reduced, the sampling speed is lowered and accordingly, the data output timing is retarded according to the sampling speed.
Shading means fluctuation in an analog image signal obtained from a photoelectric read-out means (local reduction in photo-detecting efficiency) due to unevenness in the intensity of the scanning light beam caused by unevenness in reflectance on the reflecting surface of the light deflector for deflecting the scanning light beam (e.g., polygonal mirror, galvanometer mirror or the like), fluctuation in the scanning speed caused by fluctuation in deflecting speed of the deflector, or unevenness in detection caused by unevenness in sensitivity in the main scanning direction of a photoelectric detector disposed to extend in the main scanning direction. Further the data for shading correction means shade properties which are obtained in advance, for instance, by use of a reference recording medium such as a stimulable phosphor sheet which has been uniformly exposed to a radiation. See, for instance, Japanese Unexamined Patent Publication Nos. 61(1986)-189763, 62(1987)-47259, 62(1987)-47261, 64(1989)-86759, and 2(1990)-58973.
As for the items (1) to (3) and (6) to (8) to be changed according to the desired picture element density, a plurality of beam diameters (1), a plurality of beam powers (2), a plurality of sensitivities (3), a plurality of frequency response properties (6), a plurality of frequency response properties different in cut-off frequencies (7), and a plurality of parameters for filtering processing (8) may be prepared by picture element densities and the items may be changed by selecting one of the respective properties according to the desired picture element density.
The data for shading correction (4) may be changed, as well as by the method described above where one of a plurality of sets of data is selected, by obtaining a set of data for shading for the desired picture element density by sampling a single set of data which has been prepared for said predetermined picture element density.
The picture element densities may be divided to a plurality of levels, e.g., high, standard and low, and the values of m and n may be related to the xe2x80x9chigh picture element densityxe2x80x9d, the xe2x80x9cstandard picture element densityxe2x80x9d and the xe2x80x9clow picture element densityxe2x80x9d so that when the values of m and n give a picture element density within the high picture element density, data for shading correction prepared for the high picture element density is selected, when the values of m and n give a picture element density within the standard picture element density, data for shading correction prepared for the standard picture element density is selected, and when the values of m and n give a picture element density within the low picture element density, data for shading correction prepared for the low picture element density is selected.
The data for shading correction may be selected from a plurality of sets of data for shading correction according to the set values of m and n. In this case, it is preferred that the data for shading correction be selected in one of the following manners [I] and [II]. That is;
[I] With a plurality of sets of data for shading correction which have been set by values of m and n stored in a first memory, data for shading correction corresponding to the selected values of m and n is transferred from the first memory to a second memory each time the values of m and n are selected, and the transferred data for shading correction is read out from the second memory as the selected data for shading correction.
[II] With a plurality of sets of data for shading correction which have been set by values of m and n stored in a first memory, all the sets of data for shading correction are transferred from the first memory to a second memory at different addresses by the values of m and n at a desired time such as starting of the system, and data for shading correction corresponding to the selected values of m and n is read out from the address of the second memory corresponding to the selected values of m and n as the selected data for shading correction.
When the method [I] is employed, the second memory may be small in capacity and the hardware may be simple in structure. On the other hand, when the method [II] is employed, the software may be simple in structure and the data for shading correction can be read out from the second memory at a high speed. In accordance with a third aspect of the present invention, there is provided a method of obtaining, in a method of reading out a digital image signal at a predetermined picture element density by causing a light beam to repeatedly scan a recording medium bearing thereon an image in a main scanning direction at a predetermined main scanning speed while moving the recording medium in a sub-scanning direction substantially perpendicular to the main scanning direction at a predetermined sub-scanning speed, thereby two-dimensionally scanning the recording medium with the light beam, photoelectrically detecting signal light emitted from the recording medium upon exposure to the light beam to obtain an analog image signal, sampling the analog image signal at a predetermined intervals, and quantizing the sampled values, a digital image signal at a picture element density 1/mxc3x97n times the predetermined picture element density, the method comprising the steps of
changing the sub-scanning speed to m( greater than 0) times said predetermined sub-scanning speed, changing the intervals at which the analog image signal is sampled to intervals n( greater than 0) times said predetermined intervals, and changing at least one of the following properties according to the values of said m and n.
(1) The beam diameter of the light beam.
(2) The sensitivity of detecting the signal light.
(3) The preset data for shading correction when shading correction is to be carried out on the analog image signal.
(4) The frequency transfer properties when the analog image signal is to be logarithmically amplified.
(5) The cut-off frequency when filtering for removing aliasing noise is carried out prior to sampling the analog image signal.
In accordance with a fourth aspect of the present invention, there is provided an image signal read-out system for carrying out the method in accordance with the first aspect of the present invention. That is, in accordance with the third aspect of the present invention, there is provided an image signal read-out system for reading out a digital image signal at a predetermined picture element density comprising a main scanning means which causes a light beam to repeatedly scan a recording medium bearing thereon an image in a main scanning direction at a predetermined main scanning speed, a sub-scanning means which moves the recording medium and/or the light beam relatively to each other in a sub-scanning direction substantially perpendicular to the main scanning direction at a predetermined sub-scanning speed, a photoelectric detector means which photoelectrically detects signal light emitted from the recording medium upon exposure to the light beam to obtain an analog image signal, and an A/D convertor means which samples the analog image signal at a predetermined intervals and quantizes the sampled values, thereby obtaining a digital image signal at a predetermined picture element density, wherein the improvement comprises
a sub-scanning speed changing means which causes the sub-scanning means to move the recording medium and/or the light beam relatively to each other in the sub-scanning direction at a speed m( greater than 0) times said predetermined sub-scanning speed, and a sampling interval changing means which causes the A/D convertor means to sample the analog image signal at intervals n( greater than 0) times said predetermined intervals.
It is preferred that the image signal read-out system be further provided with a picture element density changing processing means which carries out, on the digital image signal output from the A/D convertor means, picture element density changing processing for changing the number of the picture elements in the main scanning direction to a/m (a greater than 0) times and the number of the picture elements in the sub-scanning direction to a/n times.
In accordance with a fifth aspect of the present invention, there is provided an image signal read-out system for carrying out the method in accordance with the second aspect of the present invention. That is, in accordance with the fourth aspect of the present invention, there is provided an image signal read-out system for obtaining an addition image signal at a predetermined picture element density comprising a main scanning means which causes a light beam to repeatedly scan a recording medium bearing thereon an image in a main scanning direction at a predetermined main scanning speed, a sub-scanning means which moves the recording medium and/or the light beam relatively to each other in a sub-scanning direction substantially perpendicular to the main scanning direction at a predetermined sub-scanning speed, a photoelectric detector means which photoelectrically detects signal light emitted from both sides of the recording medium upon exposure to the light beam to obtain a pair of analog image signals, an A/D convertor means which samples the analog image signals at a predetermined intervals and quantizes the sampled values, thereby obtaining a pair of digital image signals, and an adder means which adds together the digital image signal and obtains an addition image signal at a predetermined picture element density, wherein the improvement comprises
a sub-scanning speed changing means which causes the sub-scanning means to move the recording medium and/or the light beam relatively to each other in the sub-scanning direction at a speed m( greater than 0) times said predetermined sub-scanning speed,
a sampling interval changing means which causes the A/D convertor means to sample the analog image signal at intervals n( greater than 0) times said predetermined intervals, and
picture element density changing processing means which carries out, on the digital image signals output from the A/D convertor means, picture element density changing processing for changing the number of the picture elements in the main scanning direction to a/m (a greater than 0) times and the number of the picture elements in the sub-scanning direction to a/n times.
In the system of the fifth aspect of the present invention, it is preferred that the picture element density changing processing means carries out said picture element density changing processing on said two digital image signals before they are added together.
Further preferably the systems of the fourth and fifth aspects of the present invention are provided with a characteristic changing means which changes at least one of the following properties according to the values of said m and n.
(1) The beam diameter of the light beam.
(2) The power of the light beam.
(3) The sensitivity of detecting the signal light.
(4) The preset data for shading correction when shading correction is to be carried out on the analog image signal.
(5) The timing at which the data for shading correction is output from a memory.
(6) The frequency transfer properties when the analog image signal is to be logarithmically amplified.
(7) The cut-off frequency when filtering for removing aliasing noise is carried out prior to sampling the analog image signal.
(8) The parameter for filtering processing in the picture element density changing processing (including the parameter for thinning the picture elements when thinning processing is carried out as the filtering processing).
The sub-scanning speed changing means and the sampling interval changing means may form a part of the characteristic changing means.
When the characteristic changing means includes a means for changing the data for shading correction, the data for shading correction may be selected from a plurality of sets of data for shading correction according to the set values of m and n. In this case, it is preferred that the data for shading correction be selected in one of the following manners [I] and [II]. That is;
[I] With a plurality of sets of data for shading correction which have been set by values of m and n stored in a first memory, data for shading correction corresponding to the selected values of m and n is transferred from the first memory to a second memory each time the values of m and n are selected, and the transferred data for shading correction is read out from the second memory as the selected data for shading correction.
[II] With a plurality of sets of data for shading correction which have been set by values of m and n stored in a first memory, all the sets of data for shading correction are transferred from the first memory to a second memory at different addresses by the values of m and n at a desired time such as starting of the system, and data for shading correction corresponding to the selected values of m and n is read out from the address of the second memory corresponding to the selected values of m and n as the selected data for shading correction.
When the method [I] is employed, the second memory may be small in capacity and the hardware may be simple in structure. On the other hand, when the method [II] is employed, the software may be simple in structure and the data for shading correction can be read out from the second memory at a high speed.
In accordance with a sixth aspect of the present invention, there is provided an image signal read-out system for reading out a digital image signal at a predetermined picture element density comprising a main scanning means which causes a light beam to repeatedly scan a recording medium bearing thereon an image in a main scanning direction at a predetermined main scanning speed, a sub-scanning means which moves the recording medium and/or the light beam relatively to each other in a sub-scanning direction substantially perpendicular to the main scanning direction at a predetermined sub-scanning speed, a photoelectric detector means which photoelectrically detects signal light emitted from the recording medium upon exposure to the light beam to obtain an analog image signal, and an A/D convertor means which samples the analog image signal at a predetermined intervals and quantizes the sampled values, thereby obtaining a digital image signal at a predetermined picture element density, wherein the improvement comprises
picture element density input means which receives values of m (m greater than 0) and n (n greater than 0) which respectively represent that the picture element density in the main scanning direction is to be changed to 1/m times that of the predetermined picture element density and that the picture element density in the sub-scanning direction is to be changed to 1/n times that of the predetermined picture element density,
a sub-scanning speed changing means which causes the sub-scanning means to move the recording medium and/or the light beam relatively to each other in the sub-scanning direction at a speed m( greater than 0) times said predetermined sub-scanning speed,
a sampling interval changing means which causes the A/D convertor means to sample the analog image signal at intervals n( greater than 0) times said predetermined intervals, and
a characteristic changing means which changes at least one of the following properties according to the values of said m and n.
(1) The beam diameter of the light beam.
(2) The sensitivity of detecting the signal light.
(3) The preset data for shading correction when shading correction is to be carried out on the analog image signal.
(4) The frequency transfer properties when the analog image signal is to be logarithmically amplified.
(5) The cut-off frequency when filtering for removing aliasing noise is carried out prior to sampling the analog image signal.
In the methods and the systems in accordance with the present invention, by changing the sub-scanning speed to m times (m greater than 0) the predetermined sub-scanning speed, the number of the scanning lines of the light beam is changed to 1/m times, whereby the picture element density in the sub-scanning direction is changed to 1/m times. Further by changing the sampling intervals to n times (n greater than 0) the predetermined sampling intervals, the picture element density in the main scanning direction is changed to 1/n times. Accordingly, the picture element density of the image signal finally obtained (the addition image signal in the case where signal light emitted from both sides of the recording medium upon exposure to the light beam is photoelectrically detected and an addition image signal is obtained by adding two digital image signals) is a/(mxc3x97n)2 times said predetermined picture element density. Further since the picture element density can be changed without changing the main scanning speed, it is not necessary to change the driving speed of the scanning optical system for scanning the light beam (e.g., a polygonal mirror or a galvanometer mirror), whereby generation of a dead time (a time which corresponds to the time necessary for the driving speed of the optical system to be stabilized and for which read-out of an image signal is impossible) can be avoided.
Further when picture element density changing processing for changing the number of the picture elements in the main scanning direction to a/m (a greater than 0) times and the number of the picture elements in the sub-scanning direction to a/n times is carried out, the picture element densities in the main scanning direction and the sub-scanning direction of the image signal finally obtained (the addition image signal in the case where signal light emitted from both sides of the recording medium upon exposure to the light beam is photoelectrically detected and an addition image signal is obtained by adding two digital image signals) are both changed to a/(mxc3x97n) times, whereby the picture element densities in the sub-scanning direction and in the main scanning direction can be changed in the same proportion and an image signal at a picture element density of {a/(mxc3x97n) }2 times the predetermined picture element density can be obtained with the aspect ratio of the image kept unchanged and without changing the main scanning speed.
Especially in the case where signal light emitted from both sides of the recording medium is to be detected (will be referred to as xe2x80x9cthe both-side readingxe2x80x9d, hereinbelow) as in the method of the second aspect of the present invention or the system of the fifth aspect of the present invention, it is necessary to slow the scanning speed as compared with the case where signal light from one side of the recording medium is to be detected in order to give energy of the light beam sufficiently to the back side of the recording medium. However even in the case of the both-side reading, the scanning time can be shortened by increasing the sub-scanning speed so long as the picture element density can be lowered.
Further, in the case of the both-side reading, by carrying out the picture element density changing processing on the two digital image signals and obtaining the addition signal by adding the processed image signals, the amount of operation to be performed when the digital image signals are added can be reduced, whereby the time required to add the digital image signals can be shortened and the processing can be carried out in a shorter time.
Further, in accordance with the present invention, various problems which arise when the reading picture element density is changed can be overcome.
That is, the aforesaid problem 1), that when the picture element density is changed, energy of signal light emitted from the stimulable phosphor sheet per one picture element differs from that for the original picture element density, and, when the difference between the changed picture element density and the original picture element density is large, the density (or brightness) of the overall image to be reproduced can be changed to adversely affect diagnostic performance of the image, can be overcome by changing (1) the beam diameter of the light beam according to the picture element density so that the amount of energy to be applied to a unit area of the recording medium is kept unchanged, or by changing (2) the power of the light beam according to the picture element density so that the amount of energy to be applied to a unit area of the recording medium is kept unchanged, or by changing (3) the sensitivity of detecting the signal light according to the picture element density so that the level of the image signal is kept unchanged.
Further the aforesaid problem 2), that when shading correction is to be carried out on the analog image signal, properties of shading to be corrected varies before and after the picture element density change and the shading sometimes cannot be properly corrected, can be overcome by changing (4) the preset data for shading correction according to the picture element density so that shading correction can be carried out following change in shading properties or by changing (5) the timing at which the data for shading correction is output from a memory so that shading correction can be carried out following change in the sampling speed.
The aforesaid problem 3), that when the analog image signal is to be logarithmically amplified, the frequency transfer properties can vary before and after the picture element density change, can be overcome by changing (6) the frequency transfer properties when the analog image signal is logarithmically amplified according to the picture element density so that the frequency transfer properties of the digital image signal obtained is kept unchanged.
The aforesaid problem 4), that when filtering for removing aliasing noise is to be carried out prior to sampling the analog image signal, aliasing noise sometimes cannot be properly cut since Nyquist frequency varies before and after the picture element density change, can be overcome by changing (7) the cut-off frequency of filtering according to the picture element density so that the frequency properties of the digital image signal obtained is kept unchanged.
Further, by changing (8) the parameter for filtering processing (the coefficient of the mask operation) in the picture element density changing processing, the aliasing noise upon picture element density change can be suppressed lower than a certain level and at the same time, error generated by interpolation processing such as spline interpolation operation can be suppressed.